The present invention relates generally to electric double layer capacitors, and more particularly to a high performance double layer capacitor made with low-resistance carbon powder electrodes.
Double layer capacitors, also referred to as electrochemical double layer capacitors (EDLC), are energy storage devices that are able to store more energy per unit weight and unit volume than traditional capacitors. In addition, they can typically deliver the stored energy at a higher power rating than rechargeable batteries.
Double layer capacitors consist of two porous electrodes that are isolated from electrical contact by a porous separator. Both the separator and the electrodes are impregnated with an electrolytic solution. This allows ionic current to flow between the electrodes through the separator at the same time that the separator prevents an electrical or electronic (as opposed to an ionic) current from shorting the cell. Coupled to the back of each of the active electrodes is a current collecting plate. One purpose of the current collecting plate is to reduce ohmic losses in the double layer capacitor. If these current collecting plates are non-porous, they can also be used as part of the capacitor seal.
Double layer capacitors store electrostatic energy in a polarized liquid layer which forms when a potential exists between two electrodes immersed in an electrolyte. When the potential is applied across the electrodes, a double layer of positive and negative charges is formed at the electrode-electrolyte interface (hence, the name xe2x80x9cdouble layerxe2x80x9d capacitor) by the polarization of the electrolyte ions due to charge separation under the applied electric field, and also due to the dipole orientation and alignment of electrolyte molecules over the entire surface of the electrodes.
The use of carbon electrodes in electrochemical capacitors with high power and energy density represents a significant advantage in this technology because carbon has a low density and carbon electrodes can be fabricated with very high surface areas. Fabrication of double layer capacitors with carbon electrodes has been known in the art for quite some time, as evidenced by U.S. Pat. No. 2,800,616 (Becker), and U.S. Pat. No. 3,648,126 (Boos et al.).
A major problem in many carbon electrode capacitors, including double layer capacitors, is that the performance of the capacitor is often limited because of the high internal resistance of the carbon electrodes. This high internal resistance may be due to several factors, including the high contact resistance of the internal carbon-carbon contacts, and the contact resistance of the electrodes with a current collector. This high resistance translates to large ohmic losses in the capacitor during the charging and discharge phases, which losses further adversely affect the characteristic RC (resistance times capacitance) time constant of the capacitor and interfere with its ability to be efficiently charged and/or discharged in a short period of time. There is thus a need in the art for lowering the internal resistance, and hence the time constant, of double layer capacitors.
U.S. Pat. No. 5,907,472 to Farahmandi et al., the complete disclosure of which is incorporated herein by reference, discloses a multi-electrode double layer capacitor having a single electrolyte seal and aluminum-impregnated carbon cloth electrodes. The use of the aluminum-impregnated carbon cloth electrodes described therein results in a double layer capacitor having a very low internal resistance. The carbon cloth used in such electrodes, however, tends to be somewhat costly. Thus, it would be advantageous to have a method and/or apparatus for lowering the internal resistance of double layer capacitors that does not rely on carbon cloth.
It is thus apparent that there is a continuing need for improved double layer capacitors. Such improved double layer capacitors need to deliver large amounts of useful energy at a very high power output and energy density ratings within a relatively short period of time. Such improved double layer capacitors should also have a relatively low internal resistance and yet be capable of yielding a relatively high operating voltage.
Furthermore, it is also apparent that improvements are needed in the techniques and methods of fabricating double layer capacitor electrodes so as to lower the internal resistance of the double layer capacitor and maximize the operating voltage. For example, the method used to connect the current collector plate to the electrode is important because the interface between the electrode and the current collector plate is a source of internal resistance of the double layer capacitor. Since capacitor energy density increases with the square of the operating voltage, higher operating voltages thus translate directly into significantly higher energy densities and, as a result, higher power output ratings. It is thus readily apparent that improved techniques and methods are needed to lower the internal resistance of the electrodes used within a double layer capacitor and increase the operating voltage.
The present invention advantageously addresses the needs above as well as other needs by providing a method of making an electrode structure for use in a double layer capacitor. The method includes the steps of: preparing a first slurry that includes conducting carbon powder and a binder; applying the first slurry to a current collector plate; drying the applied first slurry to form a primary coating; preparing a second slurry that includes activated carbon powder, a solvent and a binder; and applying the second slurry to the primary coating.
The present invention also provides a double layer capacitor that includes first and second electrode structures, a porous separator, and a saturation means. The first electrode structure includes a first current collector foil, a first primary coating formed on the first current collector foil, and a first secondary coating formed on the first primary coating. The second electrode structure includes a second current collector foil, a second primary coating formed on the second current collector foil, and a second secondary coating formed on the second primary coating. The first and second primary coatings include conducting carbon powder, and the first and second secondary coatings include activated carbon powder. The porous separator is positioned between the first and second electrodes structures. The saturation means saturates the porous separator and the first and second electrodes structures in a prescribed electrolytic solution.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized.